Poly(ADP-ribose) polymerase (PARP) is a crucial protein that is involved in repairing DNA. This protein detects single-strand DNA breaks and signals the enzymatic machinery needed for DNA repair. Specifically, PARP binds to the DNA and, using NAD+ as a substrate, synthesizes a polymeric adenosine diphosphate ribose chain, which acts as a signal for DNA=repairing enzymes. PARP is a clinically significant enzyme, because suppressing its activity with inhibitors have been shown to kill cancer cells. Because of this PARP inhibitors have been used to treat a large number of different cancers, including breast, prostate, and pancreatic cancer.
 
The PARP activity assay kit measures the activity of PARP by detecting its ability to perform ADP-ribosylation on histones. First, a plate is coated with histone. Then, a mixture of activated DNA and a biotin-tagged PARP substrate (NAD+) is added to the wells. When samples containing PARP are added to the mixture, the PARP will attach the biotin-tagged substrate to the histone through ADP-ribosylation. Once the wells are washed, the biotin-tagged substrate, which is bound to the histones attached to the plate, can be detected using streptavidin-HRP. After the addition of HRP substrate and stop solution, a colorimetric signal is generated, which can be measured at an absorbance of 450 nm with a plate reader.